Spectroscopy is one of the main compelling applications of terahertz (THz) radiation. A typical THz Spectroscopy system includes a tunable THz transmitter capable of generating THz radiation for irradiating a sample and a THz detector capable of receiving THz radiation response from the irradiated sample and providing indication (electric signal) of the strength and propagation delay of the detected radiation from the sample. For example, the technique of obtaining information related to terahertz waves that are transmitted through or reflected by a sample is disclosed in U.S. Pat. No. 7,551,269.
A tunable THz transmission device typically includes two distributed feedback (DFB) lasers and a THz emitter associated with an antenna. One or both of the lasers are associated with a controllable thermo-electric-cooling (TEC) system which controls their operating temperatures and thus their output wavelengths. The lasers are used to illuminate the THz emitter (being typically a photo-conducting element) with a light signal containing an oscillating component at the beat frequency (difference frequency) of the lasers. In the emitter, a THz frequency current is excited while applying D.C. bias to the photo-conducting element which changes conductance at the beat frequency, causing the antenna coupled to it to radiate in the THz band. The frequency of the current in the antenna (and that of the emitted radiation) is the difference between the frequencies of the lasers (beat frequency), and thus tuning the frequency of the emitted THz radiation is achieved by changing the output frequency(ies) of one or both of the lasers. Such photo-mixing based THz emitter is described for example in WO 2007/132459, assigned to the assignee of the present application.
In a THz detection device (receiver), a responding THz radiation signal, (e.g. reflected, transmitted or scattered wave) from the irradiated sample is incident upon the antenna of the receiving device, which is constructed similarly to the emitting device. This THz signal induces a voltage across the receiving photo-conductor which, in this case, has substantially zero D.C. bias component. The conductivity of the receiving photo-conductor is also modulated at the optical beat frequency by the incident laser light in the same way as the transmitter device is modulated. If the beat frequency is constant, the THz modulation of the conductance interacting with the THz bias created by the signal from the antenna generates a low frequency (e.g. D.C.) signal component proportional to the amplitude of the incident THz wave and dependent on the relative phases of the received wave and the optical beat-frequency modulation. Such arrangement acts as a homodyne mixer in which the modulating optical beat frequency used in the emitter is also used as a reference signal (i.e. reference oscillator modulation) in the receiver. The intermediate frequency is centered around the D.C. (zero frequency), and the arrangement provides coherent detection. The desired signal centered at D.C. can be extracted by using a low pass filter.
The above is schematically shown in FIG. 1. Two light beams of wavelengths/frequencies (λ1, ω1) and (λ2, ω2) respectively are combined by a fiber splitter/combiner to propagate along a combined optical path, and then are split into two light components, each of a beat frequency (ω2−ω1) propagating along spatially separated optical paths towards respectively the transmitter- and receiver-antenna units. The light component at the receiver-antenna unit serves as a reference beam or local oscillator modulation. Each of the transmitter- and receiver-antenna units includes photo-conductors with antennas. Radiation emitted by the transmitter-antenna unit is directed (by a reflector) to the sample, and a radiation response of the sample (reflection from the sample) is directed (by another reflector) to the receiver-antenna unit. The latter includes a low-pass filter which operates to extract the desired signal. A photomixing based transceiver system of the kind described above is disclosed for example in U.S. Pat. No. 6,348,683.
A major disadvantage of such arrangement is associated with the fact that amplifiers exhibit high noise density at low frequencies called “flicker noise”. Accordingly, in order to achieve reasonable signal to noise ratio, the signal (THs radiation incident onto the sample) has to be of as high as possible amplitude, and thus enabling the terahertz signal from the sample to be sufficiently strong when arriving at the receiver. Since the “flicker noise” and a detected signal (resulting from the interaction between the sample's response and reference signals) are in this case occupy frequency band with high density noise, a band pass filter (low pass filter) cannot be effectively utilized to filter out the noise. The severity of the flicker noise phenomenon is illustrated in FIG. 2 which shows noise density for a typical integrated-circuit amplifier (e.g. utilizing MAX4475 amplifier commercially available from Maxim. Integrated Products Inc). It may be seen that the noise density is rising rapidly as frequencies approach D.C. At 10 Hz the density is more than five times the density at 10 kHz, and at 1 Hz the density will be very much larger, probably several hundred time the density at 10 kHz.
Another technique of the kind specified is disclosed in U.S. Pat. No. 7,687,773. This technique relates to sub-millimeter wave frequency heterodyne imaging systems, more specifically, to a sub-millimeter wave frequency heterodyne detector system for imaging the magnitude and phase of transmitted power through or reflected power off of mechanically scanned samples at sub-millimeter wave frequencies.